Single piston calipers ( 2 piston on M5 and M6 ) apperently work best for 91% of BMW's M Customers

So thats why they fit them

Maybe I do not drive my E46 M3 as hard as some of you guys (those top 9% E90Fleet apparently fits into), but the calipers themselves have never seemed to me to be a weak point of the car. I had no issues with repeated hard braking from 120-130 for turn 5 at Road America, which is a second-gear turn at the bottom of a fairly steep hill.

All I did to "prepare" the car was to remove the brake duct panels, install stainless steel lines, and replace the pads and fluid.

I think multi piston setups are not necessary in the E46 M; no one outside BMW knows yet about the E92 of course.

To me, Brembos and similar setups are more for bling than performance in non-race applications. They are expensive, add weight, and have little real benefit under most conditions, including DE-type conditions.